Copper vs aluminum is nothing more than conduction vs convection. Where conduction is paramount, copper wins by virtue of higher conductivity. Where convection to a lousy medium (read: air) is paramount, surface area tends to rule. Even though aluminum has a lower conductivity, when you consider the ratio of density to conductivity, aluminum is actually better. All this means is that you can get a lot more surface area from a given mass of aluminum. This extra surface area makes up for the lousy conductivity.
If you have two identical shapes, copper will win. If you have two identical masses, you can generate a lot more surface area with the aluminum and win a convection-to-air competition. This is the real reason for hybrid heat sinks. The fact that aluminum is cheaper and easier to work with also explains why you'll normally find it in applications convecting heat to air.
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